The crystalline material known as YAG (yttrium aluminum garnet, with the chemical formula Y.sub.3 Al.sub.5 O.sub.12) is well known as a solid-state laser host material. When a fraction of Y atoms in YAG, generally between one and ten percent, is replaced by thulium atoms (Tm), this material, Tm:YAG, can be used to make lasers that oscillate near an optical wavelength .lambda.=2 microns (.mu.m). When tuning means, such as etalons or prisms, are placed in a laser resonator containing the Tm:YAG material, the laser output signals can be tuned over a limited range around 2 .mu.m. However, inclusion of tuning means in a laser resonator is undesirable, because this reduces the efficiency and increases the complexity of the resulting laser.
One attractive application of lasers that operate near .lambda.=2 .mu.m is for remote sensing of wind velocity and other weather parameters. This application requires use of a wavelength for which the atmosphere is highly transparent or tuning to a well-defined wavelength on or off a local maximum or local minimum for light absorption. FIGS. 1 and 2 illustrate an atmospheric absorption parameter .alpha. (km.sup.-1) for a cumulative wavelength range 2.010 .mu.m.ltoreq..lambda..ltoreq.2.030 .mu.m, showing some fine structure in this parameter for variable wavelength. From this Figure it is evident that monochromatic light with certain wavelengths is at least 50 percent attenuated within 100 meters, and that monochromatic light at other wavelengths may be only 50 percent attenuated after propagation for 10 km or more through the atmosphere. Preferably, although not necessarily, laser energy of at least 25 millijoules (mJ) should be used to propagate a distance such as 10 kilometers (km) or more through a stable atmosphere and to provide a usable return signal from aerosol scatter. Propagation of such a signal may require laser energy as high as 1-10 Joules, if the signal source is a satellite orbiting at a height of about 300 km.
Most wind-sensing laser systems used thus far have relied upon a CO.sub.2 laser (.lambda.=10.6 .mu.m) or, less often on a Nd:YAG laser (.lambda.=1.06 .mu.m). Radiation from a CO.sub.2 laser often, falls in an absorption band for atmospheric water vapor or for CO.sub.2 and thus has a relatively small transmission coefficient at such wavelengths. Further disadvantages of a CO.sub.2 laser are: (1) the size of a CO.sub.2 laser required to produce a given output energy is much larger than a corresponding solid-state laser; (2) a CO.sub.2 laser often require high drive voltages, of the order of 1000 volts, as compared with a drive voltage of about 10 volts required for a solid-state laser; and (3) a CO.sub.2 laser will often wear out quicker or require more frequent maintenance than a solid-state laser.
Use of conventional Nd:YAG laser radiation in an open environment is problematical, because light of this wavelength presents a serious hazard to the eye. S. W. Henderson recently demonstrated a coherent laser radar system using a Cr:Tm:Ho:YAG solid-state laser to produce 2.09 .mu.m radiation ("Coherent solid-state 1.06 and 2.1 .mu.m lidar system for wind velocity measurements"; invited lecture EO4.2, given at LEOS annual meeting, November 1990, Boston, Mass.). Another interesting laser material, Tm:YAG, has been shown to produce laser radiation at wavelength 2.02 .mu.m with up to 1 mJ output energy (T. J. Kane and T. S. Kubo, "Diode-pumped injection-seeded Q-switched Tm:YAG laser", invited lecture, SSL1.3, 1990 LEOS annual meeting; and P. J. M. Suni and S. W. Henderson, "Diode-pumped 2 .mu.m lasers for the mJ/pulse regime and beyond", invited lecture SSL1.1, 1990 LEOS armual meeting).
What is needed is a laser (1) whose emission wavelength is selectable across a range of wavelengths, including a wavelength interval such as 2.010- 2.030 .mu.m, where light is strongly transmitted in the atmosphere; (2) that is both simple to fabricate and relatively efficient in operation; (3) whose output can be scaled over an output energy range that includes 1-1,000 millijoules; and (4) that can oscillate at a single frequency without use of an intra-cavity wavelength selection device.